1. Field of the Invention
The present invention relates to light emitting devices, and more particularly to a light emitting device having reduced reflection of ambient light.
2. Description of the Prior Art
An organic light emitting diode (OLED) (also known as an organic electroluminescent device) is an LED that uses an organic layer as the active layer. In recent years, OLED has been gradually applied in flat panel displays and has many advantages, such as operating at low voltage, with high brightness, light weight and slim profile, full viewing angle, and highly effective contrast ratio.
However, an organic electroluminescent device easily reflects ambient light. This causes inadequate contrast ratio and poor device properties. A frequently used method of increasing the luminance contrast ratio, is adhesion of a polarizer on the light emitting face. However, this method reduces the luminance to only about 30%. In order to achieve the required luminance, the operational voltage must be greatly increased, which, however, shortens the device life.
FIG. 1 is a cross-section view illustrating an example of a conventional electroluminescent device having an anti-reflective member that minimizes reflection of ambient light. Referring to FIG. 1, first, an anti-reflective member 200 is coated on a substrate 100, and then an anode 300, a light emitting layer (EL) 400, and a cathode 500 are sequentially coated. The anti-reflective member 200 includes a reflective metal layer 210, a transparent conductive layer 220, and a thin semi-transparent metal layer 230. Based on the optical interference principle, by selecting a suitable material and thickness of each layer in the anti-reflective member, there is a phase difference between the reflected ambient light L11 by the semi-transparent thin metal layer 230 and the reflected ambient light L12 by the metal reflective layer 210, causing interference between the reflections. Thus, the luminance contrast ratio is increased, while maintaining a luminance higher than 50% of the original luminance.
In U.S. Pat. No. 6,429,451 (Hung et al.), a reflection-reducing layer made of an n-type semiconducting material is disposed between the light-reflective cathode and the electron transporting layer in an organic light emitting device. Thus, ambient light reflection from the cathode is reduced, thus increasing the contrast ratio. Suitable n-type semiconducting material can be ZnO or ZnS.